The immune system reacts to challenge in a very selective and highly defined manner. T-helper lymphocytes play a central role and by either favouring T helper 1 (Th1) or Th2 development, they orchestrate immunological responses through the release of specific cytokines. This dichotomy is particularly evident following exposure to intracellular bacteria or viruses when Th1 responses predominate and mediate protection, and during helminth infestations when Th2 responses are preferentially developed (Abbas A K et al., Nature (1996), 387, 787-793; Mosmann T R et al., Imm Rev (1991) 123, 209-229 ). A Th1 response is accompanied by production of interleukin 2 (IL-2) and gamma interferon (IFN-γ). These cytokines drive specific antibody subclasses and enhance cellular immune responses through activation of macrophages and generation of CD8 cytotoxic T-lymphocytes. A Th2 response is characterized by an increase in IL-4, IL-5 and IL-13. These cytokines stimulate increased IgE levels, induce eosinophilia and mucus secretion and are associated with allergic inflammation.
It was initially thought that Th1 and Th2 responses effectively regulate each other (Mosmann T R et al., Imm Res (1991) 10, 183-188). However, evidence from epidemiological studies and from a number of in vivo models has cast doubt on the mutual antagonisms of Th1 and Th2 cells (Rook G A et al., Immunol Today (2000), 21, 508-508; Rook et al. Springer Sem Immunopathol. (2004) 25: 237-255). Th1 effector cells rather than successfully switching off Th2-mediated allergic reactions in vivo may contribute additional immunopathology (Hansen G et al., J Clin Invest (1999), 103, 175-183). In a recent human clinical trial, administration of IL-12 to asthmatics reduced the magnitude of the Th2 response as suggested by the fall in eosinophil count, but failed to decrease the late asthmatic response to allergen (Bryan S A et al., Lancet (2000), 356, 2149-2153). In Th1-mediated conditions such as experimental allergic encephalitis (EAE) (Lafaille J J et al., J Exp Med (1997), 186, 307-312) and diabetes in non-obese diabetic (NOD) mice (Pakala S V et al., J Exp Med (1997), 186, 299-306) a superimposed Th2 response, rather than relieving inflammation, precipitates pathology (Lafaille et al., 1997; Pakala et al., 1997). Moreover, switching the immune response from predominantly Th1 to predominantly Th2 may merely result in a different but equally dangerous disease (Genain C P et al., Science (1996), 274, 2054-2057). In a treatment trial of human multiple sclerosis with a humanised anti-CD52 mAb that resulted in a partial switch from Th1 to Th2, the disease changed in nature without overt improvements, and autoimmune thyroid disease appeared, superimposed upon the modified disease (Coles A J et al., Lancet (1999), 354, 1691-1695).
On account of these discrepancies, research has focussed on identifying the cells responsible for successfully regulating both Th1 and Th2 responses, under conditions where these responses were no longer necessary. A number of candidates have been suggested including specific subsets of dendritic cells (Chan, C et al., Transplant Proc (2004), 36, 561S-569S), regulatory macrophages Mochida-Nishimura, K et al., Cell Immunol (2001), 214, 81-88) and several subsets of regulatory T lymphocytes (Treg) (Read, S and Powrie, F, Curr Opin Imnunol (2001), 13, 644-649). The latter group is particularly intriguing. Mechanisms of immunoregulation are characterized by mRNA expression and production of interleukin 10 (IL-10) and/or transforming growth factor beta (TGF-β) (Fukaura H et al., J Clin Invest (1996), 98, 70-77; Read and Powrie, 2001, Groux et al., Nature (1997) 389: 737-742). Production of these cytokines, and in particular IL-10 which is considered a hallmark of immunoregulation, along with expression of inhibitory signals such as CTLA-4 mediates immunoregulation by preventing Th1 and Th2 cell proliferation (Read and Powrie, 2001). In addition they maintain tolerance in the periphery by controlling unnecessary immune responses to common antigens, allergens or self (Cottrez et al, J Immunol (2000), 165, 4848-4853; Read and Powrie, 2001).
Pathogenesis in a number of human diseases, including allergy, intestinal bowel disease and some autoimmune conditions is a direct result of faulty immunoregulation. For example, this has become evident following the study of a rare X-linked genetic disorder of man, known as IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome) or as XLAAD (X-linked autoimmunity-allergic dysregulation syndrome), and an equivalent syndrome in mutant mice called “Scurfy”. In Scurfy mice (Brunkow, M et al., Nat Genet (2001), 27, 68-73) and in patients with IPEX/XLAAD, mutations in the transcription factor Foxp3 (Wildin, R S et al., Nat Genet (2001), 27, 18-20) impair development of Treg (Fontenot, J D et al., 2003; Hori, S et al., 2003). The pathology, at times fatal, has components of allergy, autoimmunity and inflammatory bowel disease (IBD). These are three major classes of chronic inflammatory disorders, which affect a large percentage of the human population.
Allergies, which have increased dramatically in the last few decades, are a result of excessive immune responses to aeroallergens, or allergens on skin or in the gut. The immediate explosive Th2-mediated response to allergen that causes clinical allergies originally evolved to respond to helminths. Disease is characterized by increased eosinophilia, IgE and mucus secretion to minute concentrations of allergens. These inappropriate responses can be controlled by IL-10-secreting Treg (Cottrez F et al., 2000). Therefore it is now accepted that immunoregulatory mechanisms are required to switch off allergic responses in animals and in man. Successful immunotherapy with high dose allergen works in some patients, and is accompanied by increased production of IL-10, and increased activity of immunoregulatory mechanisms (Francis J N et al., J Allergy Clin Immunol (2003), 111, 1255-1261; Nouri-Aria K T et al., J Allergy Clin Immunol (2002), 109, S171 (abs 105)).
Autoimmune diseases occur as a result of the host's immune system attacking its own tissues. It is a failure of regulation of the inherent anti-self capacity of the T cell repertoire selected in the thymus. Several of the most disabling autoimmune diseases of man are Th1-mediated and are increasing in frequency. Examples include multiple sclerosis and type 1 diabetes. Others, such as systemic lupus erythematosus and systemic sclerosis, are largely mediated by Th2 cells. A wide variety of organ-specific autoimmune disorders can be controlled by Treg expressing TGF-β and IL-10 mRNA (Seddon B and Mason D, Immunol Today (2000), 21, 95-99). They can inhibit both Th1-mediated (Cavani A et al., J Invest Dermatol (2000), 114, 295-302) and Th2-mediated autoimmunity (Bridoux F et al., J Exp Med (1997), 185, 1769-1775). Under experimental conditions, Treg engineered to secrete TGF-β downregulates both Th2- and Th1-mediated responses (Thorbecke G J et al., Cytokine Growth Factor Rev (2000), 11, 89-96).
Current evidence suggests that IBD (Crohn's disease and ulcerative colitis) is a result of a failure of immunoregulatory mechanisms to inhibit immune responses to gut content such as food or microbes (Boismenu R and Chen Y, J Leukoc Biol (2000), 67, 267-278). For instance IBD occurs in mice that lack IL-10 (gene knockout) and also in mice with severe combined immunodeficiency (SCID) that receive effector T cells (CD4+CD45RBhigh) without the appropriate Treg (CD4+CD45RBlow) (reviewed in (Asseman C and Powrie F, Gut (1998), 42, 157-158)). There is good evidence that Treg stop the inflammatory process in IBD through IL-10 (Singh B et al., Immunol Rev (2001), 182, 190-200). This cytokine is undergoing clinical trials as a treatment for IBD in animal models (Van Montfrans C et al., Gastroenterology (2002), 123, 1865-1876) and man (Braat H et al., Expert Opin Biol Ther (2003), 3, 725-731). Other members of the IL-10 family of cytokines (including IL-19, IL-20, IL-22, IL-24, IL-26, IL-28 and IL-29) might also be involved.
The inventors believe that in the rich developed parts of the world, there has been a large and simultaneous increase in the same three groups of chronic inflammatory disorders; 1) the allergies (Bach, J F, N Engl J Med (2002), 347, 911-920), 2) the inflammatory bowel diseases (for example Crohn's disease and ulcerative colitis) (Weinstock J V et al., Gut (2004) 53, 7-9), and 3) autoimmunity (for example, type 1 diabetes and multiple sclerosis) (Bach, 2002). The increases in allergies and Type 1 diabetes are precisely correlated both within Europe and outside Europe (Stene L C and Nafstad P (2001), Lancet 357, 607). The increases are occurring simultaneously whether the disease is mediated by Th1 cells (type 1 diabetes, multiple sclerosis, Crohn's disease (Elliott D E et al., Faseb J (2000), 14, 1848-1855) or Th2 cells (allergies). There is now general agreement that the underlying problem is the inefficient activation of immunoregulatory mechanisms in the developed countries. Impairment of Treg functions such as production of IL-10 may be to blame. This may be attributable to changing exposure to certain microorganisms (Rook G A et al., Springer Semin Immunopathol (2004), 25, 237-255).